Argonne data analysis of REx

EXECUTIVE OVERVIEW

Analysis of Argonne Labs benchmarks of a 2014 BMW i3-REx reveals:

• 6kW @40 mph or below - the maximum power output during ~5 minutes warm-up to 92C oil temperature at low efficiency
• 16kW @62mph or below - optimum power under 94C oil temperature, at highest efficiency
• 20kW @70 mph or below - maximum power after reaching 94C oil temperature, low efficiency

If starting on a Standard Day the car must be operated in range extender mode, the first 5 minutes should be under 40 mph. The REx may turn OFF briefly but at least the traction battery SOC can be maintained. After 5 minutes, the car can run up to 62 mph which keeps the REx in peak efficiency mode, maximum MPG. Above 62 up to 70 mph, the REx can provide enough power but in an inefficient power range. It may make more sense to cruise at 60-62 mph to minimize dipping into the inefficient, high-power modes.

Analysis

Argonne Labs has a downloadable test file, "61504063 Test Data.txt", embedded in a ZIP file. File 61504063 is described "Hwyx2 with Coastdowns Ph 2" covers 10.24 miles, 50.4 MPG (bag measured), burning 0.203 gallons (US) of straight gas. The test has three phases: (1) cold-start, 600 seconds; (2) 100 seconds stopped, and; hot-start, 600 seconds. During the hot-start test, the exhaust gas is collected in a bag and analyzed to calculate the fuel efficiency and emissions:

The data includes the REx-generator amps and traction battery voltage, the electrical output of the REx. It also incudes the fuel "cc/sec" which can be used to calculate the grams per second, "g/sec", and gallons per hour, "gal/hr".

Brake Specific Fuel Consumption is the ratio of g/hr over kWh. Doing the math, here is what happened during this test:

Lower BSFC is more efficient so we can see a significant difference between the first, cold-start phase, and
the following warm-start cycle. But another chart, BSFC as a function of power output reveals:


So it looks like during warm-up, there are two power bands, ~4kW and ~6kW, and at 305-310 g/kWh, inefficient. After warm-up, 20.5kW is the maximum power output but at 320 g/kWh, the least efficient. The optimum power output, 17kW, at 270 g/kWh provides the maximum range.

• NOTE: this suggests a benchmark at 60-62 mph may result in the maximum, highway speed, range. Certainly enough information to suggest a test.

Another efficiency metric used in emergency power generators is the kiloWatt hours per gallon:

Here is an earlier chart from my Prius UPS project:


Now our best guess of BMW i3-REx overhead is about 440W. It has a documented 2.5kW from the 12V DC-to-DC converter. This fixed overhead has to be part of any efficiency and power calculation. But back of the envelope:

• 2.0kW + 0.5kW - maximum 12V buss power
• 2.0kW * 80% ~= 1.6kW sustained 120VAC, sine wave, power
• 1.6kW / 120VAC ~= 13A
• 2.5kW / 10.5kW/gal ~= 0.25 gal/hr
• 2gal / 0.25 gal/hr ~= 8 hrs before refueling

We don't know how REx warm-up works when the REx cycles. However, it appears to be a practical emergency power source. There are more design issues to resolve before hacking the car.

Bob Wilson
Huntsville, AL

While an interesting exercise, don't you really think that BMW optimized the vehicle already? Certainly, not everyone's use pattern will be the same, but since driving conditions are rarely identical, a good compromise seems to be available out of the box already.

Ok, something is wrong. I ran a benchmark this morning, and got 40.3 MPG, not the expected 50 MPG:

I was expecting 1.466 gallons and instead consumed 1.817, an additional 0.351 gallons or ~24% more than expected.

Protocol

• Ran errands until SOC < 60%
• Filled with Shell 89 octane
• Enabled charge sustaining mode
• Drove 5 minutes, ~4 miles, at 40 mph, for warm-up
• Drove ~35 miles to Scottsboro AL at 61 mph
• Drove back to Shell gas station at 62 mph and filled with 89 octane

At cruise speeds 61-62 mph, the Scangauge showed 40-45 MPG with a rare 50 MPG when descending a grade. Absent other sources, we are not seeing the same MPG as expected by the model. The warm-up, 4 miles, was short and slow enough, we should not have seen a significant impact. We are burning potentially E10 but the loss of efficiency is not consistent with the results. This leaves two hypothesis: W20 engine or generator have one or more problems.

One of the subtle defects discovered in the original, 1.5L Prius was the effect of overfilling the engine oil. Usually this was induced by the dealer overfilling the engine, selling 5 quarts for the 3.7 quart engine. That remains a problem today and more than a few of us have had to drain the excess oil and seen a dramatic improvement. For example, our 2003 Prius MPG went from 45 MPG to 55 MPG after draining an extra quart:

It is certainly worth checking the oil and considering doing my own oil change using a high-quality oil.

It there is a generator problem, it would most likely be with Range Extender Machine Electronics (REME.) This subsystem converts 3-phase-to/from-DC. Although not likely, it is possible that one leg of the 3-phase converter has failed and somehow not been detected. But 'one change at a time', we need to rule out the W20 engine.

Bob Wilson

jadnashuanh said:

While an interesting exercise, don't you really think that BMW optimized the vehicle already? Certainly, not everyone's use pattern will be the same, but since driving conditions are rarely identical, a good compromise seems to be available out of the box already.

Click to expand...

I'm not trying to re-engineer the car, yet. Rather I'm trying to understand the performance.

Right now, I am concerned about REx operation at 65 mph returning 40 MPG instead of 50 MPG. The Argonne Labs version got 50 MPG so I should too. But I've now done two benchmarks, one at 65 mph and the other at 61 mph and both are not meeting the Argonne Labs MPG.

I am going back to the Argonne data to make sure I didn't miss something. I'll start with the warmed-up section at seconds 700 and see if there is something obvious.

Bob Wilson

Try premium fuel verses regular. The engine wants RON 91 or greater...anything less, it has to run detuned and thus, less efficiently. In an engine designed for it, lower octane gas is a false economy.

Happy day!

I plotted the Argonne data over time, the second pass:

Between seconds 200-275, the speed was fairly constant, 63-65 mph, and the SOC did not significantly change. Their measured 40 MPG matches my field data. Both their car and mine are working the same.

One of the things common between the Prius and BMW i3-REx is when there is a significant battery drain, the effective MPG is temporarily increased. But it is a short-term boost. Notice at seconds 400, the car began charging the traction battery and there was a step decrease in MPG.

Bob Wilson

Bob. That's a very interesting second pass graph. Thanks very much. David

jadnashuanh said:

Try premium fuel verses regular. The engine wants RON 91 or greater...anything less, it has to run detuned and thus, less efficiently. In an engine designed for it, lower octane gas is a false economy.

Click to expand...

You will see no benefit from running an octane rating higher than the REx was designed for and in fact it can cause problems with excess carbon build up due to incomplete combustion.

I believe US octane is measured totally differently to the rest of the world so cant be compared by numbers.
Octane rating is generally measured as either RON (Research Octane Number) or MON (Motor Octane Number) with slightly varying figures between the two. Here in NZ, we use RON as our standard rating, the US uses a figure called AKI (Anti Knock Index) which is derived as the average of RON and MON.
In practical terms this means that fuel rated 85-87 Octane in the US is equivalent to 91 here and US rated 93 octane is equivalent to our 98.

The file you have chosen to analyse is the US06 cycle which "was developed to address the shortcomings with the FTP-75 test cycle in the representation of aggressive, high speed and/or high acceleration driving behavior, rapid speed fluctuations, and driving behavior following startup. SFTP US06 is a high speed/quick acceleration loop that lasts 10 minutes, covers 8 miles (13 km), averages 48 mph (77 km/h) and reaches a top speed of 80 mph (130 km/h). Four stops are included, and brisk acceleration maximizes at a rate of 8.46 mph (13.62 km/h) per second. - wiki"

Hi,

janner said:

The file you have chosen to analyse is the US06 cycle . . .

Click to expand...

We are limited to the files that Argonne publishes: http://hiwaay.net/~bzwilson/BMW/argonne.html

Bob Wilson

Agreed, but for the ReX there are three 'Urban Dynamometer Driving Schedules" all of which are less onerous than the US06 'hooner' cycle.

Hi,

janner said:

Agreed, but for the ReX there are three 'Urban Dynamometer Driving Schedules" all of which are less onerous than the US06 'hooner' cycle.

Click to expand...

I loaded '61504059 Test Data' and found only 120 seconds of usable data. The reason is I only analyze steady-state data. The reason is incorporation of inertial effects in a spreadsheet is an invitation to madness. Regardless, I was able to find:

• 315.15 g/kWh - BSFC @2,400 rpm
• 4.6 kW - at ~35 mph

I need to replicate my earlier, 35 mph benchmark, 46 MPG, for a longer interval. With more miles, the accuracy improves. Although backing off to say 33 mph would provide space for the vehicle overhead. But I may wait for cooler weather to minimize any traction battery, cooling effects.

Bob Wilson